In humic-rich freshwaters, phosphorus is absorbed by high-molecular-weight HS fractions. Photolysis of these complexes leads to a slow release of P as one major regeneration mechanism of phosphorus within the illuminated water column (Figure 6). This release is based on the photoreduction of Fe(III) in the presence of dissolved chromophoric organic carbon compounds (Figure 7). The few available budget calculations demonstrate that the photolytic regeneration pathway of bioavailable P supports a medium production.
For a long time, it has been considered unlikely that chromophoric organic substances can be N sources for the microbial web. However, recent studies show that exposure of chromophoric organic substances to sunlight forces the release of nitrogen-rich compounds of high bioavailability. These compounds include NH^, which is the most effectively released, and amino acids (Figure 6).
The photoproduction of NH^ and organic N-com-pounds depends on dissolved organic N contents, rather than aromaticity of the organic carbon. At least 20 organic N-compounds were identified upon radiation of water samples containing HS. Among the amino acids identified are alanine, asparagine, citrulline, glutamic acid, histidine, nor-valine, and serine. Free amino acids are subsequently subjected to further breakdown to NH^ due to strong UV radiation and long-term exposure - which may be one mechanistic reason for the frequently observed low concentrations of free amino acids. Terrestrial chromophoric organic carbon and its photolytic degradation products, including N compounds, are also subject to long-range transport; they can not only be found in freshwater systems, but also far off the shore in oceans and support phytoplank-ton growth up to several hundred kilometers from the river delta.
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